In the refining of lignocellulose-containing fibers by, for example, a disc refiner or a conical refiner at a low consistency of about 3 to 4%, the structure of the fiber wall is loosened, and fibrils or so-called fines are detached from the surface of the fiber. The formed fines and flexible fibers have an advantageous effect on the properties of most paper grades. In the refining of pulp fibers, however, the aim is to retain the length and strength of the fibers. In post-refining of mechanical pulp, the aim is partial fibrillation of the fibers by making the thick fiber wall thinner by refining, for detaching fibrils from the surface of the fiber.
Lignocellulose-containing fibers may also be disintegrated into smaller parts by detaching fibrils which act as components in the fiber walls, wherein the particles obtained become significantly smaller in size. The properties of so-called nanofibrillar cellulose thus obtained differ significantly from the properties of normal pulp. It is also possible to use nanofibrillar cellulose as an additive in papermaking and to increase the internal bond strength (interlaminar strength) and tensile strength of the paper product, as well as to increase the tightness of the paper. Nanofibrillar cellulose also differs from pulp in its appearance, because it is gel-like material in which the fibrils are present in water dispersion. Because of the properties of nanofibrillar cellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.
Nanofibrillar cellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of nanofibrillar cellulose, the most promising potential raw material is raw material derived from plants and containing cellulose fibers, particularly wood and fibrous pulp made from it. The production of nanofibrillar cellulose from pulp requires the decomposition of the fibers further to the scale of fibrils.
The production of nanofibrillar cellulose from cellulose fibers of the conventional size class has been implemented by disc refiners of laboratory scale, which have been developed for the needs of food industry. This technique requires several refining runs in succession, for example 2 to 5 runs, to obtain the size class of nanocellulose. The method is also poorly scalable up to industrial scale.
Fibrous raw material may be disintegrated to the level of nanofibrillar cellulose by homogenization. In this process, a cellulose fiber suspension is passed several times through a homogenization step that generates high shear forces on the material.
In practice, compromises have to be made in the homogenization upon producing nanofibrillar cellulose: for good fibrillation, high input power/pulp flow rate is needed, which, in turn, decreases the productivity with the available homogenizer power and requires excessive shearing energy. It is, for example, known to pass pulp several times through a homogenizer, to achieve a desired degree of fibrillation. Another problem with the processing of fiber-containing pulp is the susceptibility of homogenizers to clogging due to their structure, which may occur already at relatively low consistencies (1-2%). Untreated native cellulose may damage the valves and other mechanical parts of the homogenizer device.